Bile Synthesis, Bile Acids, and Bile Salts

Bile is primarily composed of water (over 95%), followed by bile acids, bile salts, bilirubin, cholesterol, and various ions. It is synthesized by hepatocytes in the liver, stored in the gallbladder, and used in the small intestine.

Bile acids and salts are essential for emulsifying dietary fats into manageable pieces for lipase enzymes and for the absorption of lipophilic substances, including cholesterol, phospholipids, and fat-soluble vitamins (A, D, E, K).

The liver synthesizes primary bile acids (cholic acid and chenodeoxycholic acid) from cholesterol, using the CYP7A1 enzyme as the rate-limiting step. Intestinal bacteria later convert these into secondary bile acids (deoxycholic acid and lithocholic acid).

In the liver, bile acids are conjugated with amino acids glycine and taurine to become bile salts. About 95% of these are reabsorbed in the ileum and returned to the liver via the portal vein, a process known as enterohepatic circulation.

When fat enters the duodenum, the hormone CCK triggers gallbladder contraction and pancreatic enzyme release. Bile salts emulsify fat globules, allowing lipase to break them down into smaller micelles for absorption into the lymphatic system as chylomicrons.